Processes for the treatment of a sour hydrocarbon fraction where the fraction is treated by contacting it with an oxidation catalyst and an alkaline agent in the presence of an oxidizing agent at reaction conditions have become well known and widely practiced in the petroleum refining industry. These processes are typically designed to effect the oxidation of offensive mercaptans contained in a sour hydrocarbon fraction to innocuous disulfides-a process commonly referred to as sweetening. The oxidizing agent is most often air. Gasoline, including natural, straight run and cracked gasolines, is the most frequently treated sour hydrocarbon fraction. Other sour hydrocarbon fractions which can be treated include the normally gaseous petroleum fraction as well as naphtha, kerosene, jet fuel, fuel oil, and the like.
A commonly used continuous process for treating sour hydrocarbon fractions entails contacting the fraction with a metal phthalocyanine catalyst dispersed in an aqueous caustic solution to yield a doctor sweet product. The sour fraction and the catalyst containing aqueous caustic solution provide a liquid-liquid system wherein mercaptans are converted to disulfides at the interface of the immiscible solutions in the presence of an oxidizing agent--usually air. Sour hydrocarbon fractions containing more difficult to oxidize mercaptans are more effectively treated in contact with a metal chelate catalyst dispersed on a high surface area adsorptive support--usually a metal phthalocyanine on an activated charcoal. The fraction is treated by contacting it with the supported metal chelate catalyst at oxidation conditions in the presence of an alkaline agent. One such process is described in U.S. Pat. No. 2,988,500. The oxidizing agent is most often air admixed with the fraction to be treated, and the alkaline agent is most often an aqueous caustic solution charged continuously to the process or intermittently as required to maintain the catalyst in the causticwetted state.
The prior art shows that the usual practice of catalytically treating a sour hydrocarbon fraction containing mercaptans involves the introduction of alkaline agents, usually sodium hydroxide, into the sour hydrocarbon fraction prior to or during the treating operation. See U.S. Pat. Nos. 3,108,081 and 4,156,641. The prior art also discloses that quaternary ammonium compounds can improve the activity of these catalytic systems. For example, see U.S. Pat. Nos. 4,290,913 and 4,337,147. In these patents the catalytic composite comprises a metal chelate, an alkali metal hydroxide and a quaternary ammonium hydroxide dispersed on an adsorptive support.
The prior art also discloses the use of other nitrogen-containing compounds as promoters for mercaptan sweetening. For example, U.S. Pat. No. 4,207,173 discloses the use of guanidine as a promoter for mercaptan oxidation. Further, U.S. Pat. No. 4,753,722 discloses a large number of nitrogen-containing compounds as promoters. These compounds are classified as heterocyclic compounds, substituted homocyclic compounds and aliphatic compounds.
In contrast to this prior art, applicants have found that a dipolar compound can greatly promote the oxidation of mercaptans in both liquid-liquid and fixed bed processes. A dipolar compound is an organic compound which has a positively charged atom and an electronegative group in the same structure.
The dipolar compounds of this invention can have the structural formula ##STR1## where Z is nitrogen or phosphorus, R, R.sub.1, R.sub.2, R.sub.3, and R.sub.4 are groups as defined herein and X is halogen or hydroxide.
A preferred class of dipolar compounds are betaines which have the general formula EQU (R').sub.3 N.sup.+ CH.sub.2 COO.sup.31
where R' is an alkyl, alkaryl, aralkyl and cycloalkyl group. An especially preferred dipolar compound is ephedrine which has the formula ##STR2## and in which the hydroxyl group is capable of being deprotonated. There is no mention in the prior art that such dipolar compounds would be effective promoters for the oxidation of mercaptans. Further, applicants have found that the dipolar compounds are much more active promoters than quaternary ammonium compounds.
It is noted that these dipolar compounds superficially appear to resemble quaternary ammonium compounds (when Z is nitrogen), especially structure (A). However, there are several differences between these dipolar compounds and quaternary ammonium salts. First, the electronegative group on the dipolar compound is covalently attached to the remainder of the compound, i.e., it is a functional group, whereas the quaternary salt has a positive ion and a negative ion which are held in close proximity by ionic forces. Therefore, the negative ion in the quaternary salt can be removed and exchanged with a resin or other chemical means. An example of this type of exchange is shown by the following equation: EQU R.sub.4 N.sup.+ Cl.sup.- +Resin-OH.fwdarw.R.sub.4 N.sup.+ OH.sup.- +Resin-Cl
In contrast to this, the electronegative group on the dipolar compound cannot be exchanged by a resin or chemical means. The only way to remove the electronegative group is to break a covalent bond.
Second, the association between negative and positive ions in a salt are different from that found between the negative and positive groups in the dipolar compound. The dipolar compound may have the positive and negative groups physically separated and acting independently, whereas in a quaternary salt the counter ion must be in close proximity to balance the positive charge.
Third, the counter ions of the quaternary salts are relatively inert in catalysis reactions, whereas the electronegative groups present on the dipolar compounds can be catalytically active. That is, it makes very little difference whether the counter ion is chloride, hydroxide, acetate, etc., whereas changing the electronegative group from OH to COOH or SO.sub.3 H can effect the properties of the dipolar compound.
However, even if the dipolar compounds were considered to be structurally equivalent to a quaternary salt, the prior art does not provide any hint or motivation to add an electronegative group as a functional group to any of the groups attached to the nitrogen atom. Further, there is no indication in the prior art that a quaternary ammonium compound containing an electronegative group would be a better promoter than a quaternary ammonium compound without an electronegative group. Applicants are the first to have discovered this unexpected result.